In the plastic molding arts, virgin plastic which is to be molded is commonly used in the form of small plastic pellets. These pellets are conveniently formed by extruding plastic (typically a thermoplastic such as polyethylene) through a die to form a strand which is then severed into small lengths. For convenience, the dies are usually circular and extrusion produces cylindrical strands. However, other shapes of the small plastic pellets are known. After pelletizing, the moldable plastic can be shipped and stored in pellet form until the time of use. Pelletizing makes the plastic easy to handle and eliminates dust problems associated with powdered plastics. At the time of use, the pellets are melted and the resulting liquid plastic mass is molded and hardened.
As illustrative of the prior art related to the pelletizing of plastics, see the following U.S. Pat. Nos. 2,464,746 2,198,701 and 3,509,247.
When colored plastic articles are desired, a small amount of pigment or other coloring agent (e.g. 1% of coloring agent based upon the weight of the plastic) is added to the virgin plastic pellets and the two are mixed together before or at the time the pellets are melted and molded. For convenience, it is common in the plastic molding industry to use coloring agents in the form of color concentrates. These color concentrates can be formed by pre-blending large amounts of coloring agent with a relatively small amount of plastic (e.g. a thermoplastic such as polyethylene) so that the resulting color-rich plastic can be extruded and pelletized in the ordinary manner. In this way, the dust problems and other problems (measuring problems) associated with the use of pure coloring agents can be avoided.
In large commercial operations, it is common to mix large amounts of uncolored virgin plastic pellets with a color producing amount of color concentrate pellets to provide an ample constant source of supply to the plastic molding operations. However, if the pellets are incorrectly mixed, or if it is desired to change to a different color of plastic, unused mixtures of uncolored plastic pellets with color concentrate pellets which are of the old color are considered, and sold or disposed of as, "scrap" and are often used only in the manufacture of low priced products for other purposes. In any event, these obsolete mixtures must be stored until they can either be used or disposed of as scrap. This is because there is no commercially economical method for separating the color concentrate pellets from the uncolored plastic pellets. The two types of pellets are generally virtually identical in every way except color.
It is also common to mix different types of pellets when the types differ as to other characteristics besides color, e.g. filled vs. unfilled plastic, treated vs. untreated plastic, etc. Here again, incorrect or obsolete mixtures are likely to be "scrap".